add header for external shutdown button + halted led

[garlick]

It would be nice for embedded projects to have the ability to shutdown the pi without a keyboard.

It's possible to make a switch closure on one of the GPIO lines look like someone pressed the poweroff button. There is also a device tree overlay shipped in raspbian that lets a GPIO change state when the pi is halted. The idea is you press the shutdown button, the OS shuts down, and when it enters the "poweroff" state, the halted LED glows (or reverse logic on the LED and make it a "running" button).

If we think we might be producing a version of the board with a real time clock (#43) that changes the parport wiring, we might consider adding this as well.

I wrote this feature up in a different project in a slightly different context.